Electroluminescent device



April 12, 1960 T. JAY, .JR 2,932,746

ELECTROLUMINESCENT DEVICE Filed Feb. 25, 1957 /g O MTE INVENTOR United States Patent O 2,932,746 ELECTROLUMINESCENT DEVICE Theodore Jay, Jr., Tenaly, NJ., asignor, by mesne assignments, to Sylvania Electric Products Inc., Wilming- V ton, Del., a corporation of Delaware Application February 25, 1957, Serial No. '641,982 Claims.V (Cl. Z50-213) My invention is directed toward electroluminescent image display devices.

Certain types of phosphors, when placed in an electric Ield, will luminesce, the intensity of the emitted light being some function of the strength of this applied eld. Consequently, lms or layers containing such phosphors can be used to transform electrical energy to light energy. Phosphors of this type are said to be electroluminescent.

A first type of electroluminescent layer is formed solely from electroluminescent phosphor and is termed a homogeneous layer. A second type of electroluminescent layer is formed from a suspension of electroluminescent powders in dielectric media as described, for example, in the copending patent application Serial No. 306,909 filed August 28, 1952, by Norman L. Harvey.

Conventionally, an electroluminescent layer of either type is subtended and electrically connected `between first and second transparent electrically conductive ilms, thus forming anelectroluminescent cell. A voltage is applied between the two lms, and the cell luminesces in accordance with the magnitude of the `applied voltage. When the rst type of layer is used, a constant or variable voltage can be applied across the cell; when the second type of layer is used, the applied voltage must be a variable voltage.

An electroluminescent film can be interposed between first and second mutually perpendicular (for example, horizontal and vertical) arrays of parallel, separated electrical conductors to form a crossed-grid structure. In such a structure the lm is divided into small sections or cells," each of which is situated between one horizontal conductor and one vertical conductor. It is known that applying a suitable electric potential dilerence between the pair of conductors associated with any given cell will cause that cell to luminesce.

Such applied potentials can be switched or commutated between different pairs of conductors, to successively energize each cell in turn, thus producing an effect analogous to scanning in a cathode ray tube. If a signal carrying image information is applied to each energized `cell in turn, the combined action of the signal and the scanning effect on the crossed-grid structure will cause the structure to display an image in a manner analogous to a cathode ray tube.

In my copending application, Serial No. 641,981 tiled February 25, 1957, I disclose an improved electroluminesccnt cell having increased light output as compared to conventional cells. This improved cell comprises a homogeneous electroluminescent layer, a heterogeneous electroluminescent layer, a photoconductive layer, and lirst, second and third electrically conductive lms. The lirst lilm is in electrical Contact with one side of the homogeneous layer;,the second lrn is in electrical contact with one side of the photoconductive layer and one side of the heterogeneous layer; the third lm is in electrical contact with the other side of the heterogeneous layer. The other sides of the homogeneous layer and the photoconductive layer are in electrical contact with each other.

V-marily across the A direct voltage is applied between the first and second films, and an amplitude varying electrical signal is applied between the second and third films. As a consequence, the heterogeneous layer is energized, its light output varying in accordance with the amplitude variations of the applied signal.

When the heterogeneous layer does not emit light, the voltage drop across thehomogeneous and photoconductive layers produced by the direct Voltage appears priphotoconductive layer and the homogeneous layer does not luminesce appreciably.

When the heterogeneous layer does emit light under the influence of the applied signal, this emitted light will irradiate the photoconductive layer, thus reducing its resistance. This reduction in resistance decreases the voltage drop across the photoconductive layer and, consequently, increases the voltage drop across the homogeneous layer to a point at which the homogeneous layer will emit light. The light output from the homogeneous layer varies with the light output from the heterogeneous layer and thus varies in accordance with the applied signal.

In a sense, the portion of the structure subtended between the first and second conductive films can be regarded as analogous to an anode-cathode circuit of a vacuum tube amplifier, while the portion of the structure subtended between the second and third conductive films can be regarded as analogous to the grid-cathode circuit for this amplifier.

The light output of the improved cell thus far described, -while acceptable for applications requiring only moderate amounts of light, is insulicient for many applications. More particularly, in this cell the dark resistance of the photoconductive layer is dependent upon the thickness of this layer; since the dark resistance must be high relative to the resistance of the homogeneous electroluminescent layer, the photoconductive layer must be relatively thick. However, in this type of amplier, most of the light incident upon the surface of the photoconductive layer is absorbed in the surface itself. With the layer thickness required, the percentage change in resistance, resulting from Variations in light emitted from the heterogeneous electroluminescent layer is relatively small and limits the light output.

As further disclosed in the above identied application, Serial No. 641,981, the light output of the amplier is increased by matching the homogeneous and photoconductive layers in the manner taught in the, copending application of Gertrude Rothschild, Serial No. 619,729, liled November l, 1956, now Patent No. 2,915,- 641; i.e. the homogeneous layer has a wavelength dependent light emission characteristic at which the emission attains a maximum value for a given range of wavelengths and the photoconductive layer has a wavelength dependent photoconductive sensitivity characteristic at which the sensitivity of the layer as a whole attains a maximum value over the same given range of wavelengths. In this arrangement, as explained in more detail in the Rothschild patent application Serial No. 619,729, the dark resistance of the photoconductive layer can be made high relative to the resistance of the electroluminesccnt layer by appropriate adjustment of the thickness of the photoconductive layer. Further, when this relationship of the light emission characteristics of the electroluminescent layer to the photoconductive sensitivity characteristic of the photoconductive layer is established, the photoconductive action is not coniined to the surface of the photoconductive layer, but rather extends substantially throughout the entire layer.

I have succeeded in modifying thisimproved cell in such manner as to produce a crossed-grid structure. This structure as compared to conventional structures has greatly increased light output and further permits the asentar;

In accordance with the principles of my invention, I v

provide an electrolurninescent structure having in the order named, a first array of separated, parallel, transparent electrical conductors; a first homogeneous electroluminescent layer; a photoconductive layer; a second array of Separated, parallel, transparent electrical conductors oriented at some angle other than zero with respect to the first array conductors; a transparent insulating film; a first transparent electrically conductive film; a second heterogeneous electroluminescent layer; and a second conductive film.

Preferably, the light adsorption properties of the photoconductive layer are matched with the light emission properties of the firstelectroluminescent layer in the manner taught by Rothschild, patent application Serial'No. 619,729, now Patent No. 2,915,641.

The first and'seeond arrays, together with the first electroluminescent layer and the photoconductive layer, form a crossed-grid structure. Each conductor in the first array crosses over each conductor in the second array to form a cross-over point thereat.

Conventional switching or commutation means coupled to the conductors of both arrays serveto selectively condition each cross-over point in turn. By the term condition I mean that a D.C. potential is applied bcv tween the two conductors forming the cross-over point to be conditioned. In the absence of light excitation of the photoconductive layer, the dark resistance of the photoconductive layer and the value of the D.C. bias are such excited but is biased almost to the point of excitation.

An A C. signalis applied between the first and second conductive films and energizes (weakly or strongly) the entire second electroluminescent layer. The light emitted from this second layer irradiates the photoconductive layer, thus reducing its resistance and causing the first v electrolurninescent layer to luminesce in the region of the conditioned cross-over point. The intensity of the light output from the luminescent portion of the first layer will vary in accordance with the AC. signal but will be at a higher level of intensity of the light emitted from the second electroluminescent.layer.

My invention will now be described in detail with ref'- erence to the accompanying figure which is a partially ing film 58; a first transparent electrically conductivefilm 60; a second heterogeneous electroluminescentflayer-62; and a second electrically conductive film 64. i

Direct voltage switching or commutating potentials are applied sequentially across terminals i2-456, 42-54, 42-525 44-46, etc. to sequentially condition each crossover point for conduction in the manner previously dethat the portion of the first electroluminescent film in the neighborhood of the conditioned cross-over point is not scribed. The apparatus for producing such potentials is known and consequently is not shown in the figure.

An amplitude modulated signal is applied across terminals 10, thus actuating the entire second electro-luminescent layer, the second layer luminescing to an extent dependent upon the magnitude of the signal. The light emitted from the second layer irradiates the entire photoconductive layer and decreases its resistance. However, as a result of the conditioning of each cross-over point, the light output from the first electroluminescent layer is confined to a localized region about the particular conditioned cross-over point.v

It will beapparent from the foregoing that the switching or commutation potentials act only upon the crossedgrid section of my device, while the signal is always supplied across the terminals it). Thus, the signal and these potentials are electrically isolated from each other and yet optical interaction is obtained.

When the light output of the homogeneous layer can be relativelyl low, the photoconductive layer need not be used. In this situation, the homogeneous layer itself, rather than the series combination of the homogeneous and photoconductive layers, is conditioned by the switching potentials. Y

While I have shown and pointed out my invention as applied above, it will be apparent to those skilled in the art that many modifications can be made-Within the scope and sphere-of my'invention as defined in the claims structure interposed between and in electrical contact Y with said first and second arrays, said structure including', in series connection a first homogeneous electroluminescent element and a second photoconductive element, said first element having a wavelength-dependent light emission characteristic at which the emission attains a maximum value for a given range of wavelengths, said secand element having a wavelength-dependent phtotoconductive sensitivity characteristic at which the sensitivity attains a maximum value over substantially the same saidv range and furtherhaving a fundamental absorption edge definedl by a wavelength smaller than any Wavelength in said range; r'a transparent insulating tilm applied over said first array; and an electroluminescent cell applied over said insulating film, said cell including a third heterogeneousY electroluminescent layer electrically connected to and interposed between rst and second electrically conductive lms,

2. In combination, a crossed-grid structure; an electroluminescent cell; and a transparent electrically insulating filminterposed between said cell and said structure. r

3. In combination, a crossed-grid structure provided withrfirst and second spaced'coplanar arrays extending 'inrespectivefirst and second non-parallel direction, each array being composed of' a plurality of parallel, separated, electrtical conductors, and an element interposed between and in lelectrical contact with said arrays, said elementl including in, series connection a homogeneous electroluminescent lai/6 1' anda photoconductive layerf anelectroluminescent cellprovided` with a heterogeneous, electroluminescent layer, opposite sides of saidY heterogeneous-layerbeing coated with an electrically conductive,-

array being composed of a plurality of transparent, parallel, separated electrtical conductors, and a structure interposed between and in electrical contact with said arrays, said structure including in series connection a homogeneous electroluminescent layer and a photoconductive layer; an electroluminescent cell provided with a heterogencous electroluminescent layer, opposite sides of said heterogeneous layer being coated with an electrically conductive film, at least one of said ilms being transparent; and a transparent insulating film interposed between said cell and said structure, said insulating film making physical contact with one of said arrays and said transparent conductive films.

5. The combination as set forth in claim 4 further including means coupled to said arrays to supply switching potentials thereto and means to apply an amplitude modulated signal between said two conductive lms.

tile of this patent UNITED STATES PATENTS 2,594,740 DeForest et al. Apr. 29, 1952 5 2,698,915 Piper Jan. 4, 1955 2,791,723 Nagy et a1. May 7, 1957 2,816,236 Rosen Dec. 10, 1957 2,835,822 Williams May 20, 1958 2,915,641 Rothschild Dec. 1, 1959 10 OTHER REFERENCES A Solid-State Image Intensier by Orthuber and Ullery, Journal of the Optical Soc 15 No. 4, 297-299, April 1955.

iety of America, vol. 44, 

1. IN COMBINATION, A FIRST ARRAY OF PARALLEL, SEPARTED ELECTRICAL CONDUCTORS EXTENDING ALONG A FIRST DIRECTION, A SECOND ARRAY OF PARALLEL, SEPARATED ELECTRICAL CONDUCTORS EXTENDING ALONG A SECOND AND DIFFERENT DIRECTION, A STRUCTURE INTERPOSED BETWEEN AND IN ELECTRICAL CONTACT WITH SAID FIRST AND SECOND ARRAYS, SAID STRUCTURE INCLUDING IN SERIES CONNECTION A FIRST HOMOGENEOUS ELECTROLUMINESCENT ELEMENT AND A SECOND PHOTOCONDUCTIVE ELEMENT, SAID FIRST ELEMENT HAVING A WAVELENGTH-DEPENDENT LIGHT EMISSION CHARACTERISTIC AT WHICH THE EMISSION ATTAINS A MAXIMUM VALUE FOR A GIVEN RANGE OF WAVE LENGTHS, SAID SECAND ELEMENT HAVING A WAVELENGTH-DEPENDENT PHOTOCONDUCTIVE SENSITIVITY CHARACTERISTIC AT WHICH THE SENSITIVITY ATTAINS A MAXIMUM VALUE OVER SUBSTANTIALLY THE SAME SAID RANGE AND FURTHER HAVING A FUNDAMENTAL ABSORPTION EDGE DEFINED BY A WAVELENGTH SMALLER THAN ANY WAVELENGTH IN SAID RANGE, A TRANSPARENT INSULATING FILM APPLIED OVER SAID FIRST ARRAY, AND AN ELECTROLUMINESCENT CELL APPLIED OVER SAID INSULATING FILM, SAID CELL INCLUDING A THIRD HETEROGENEOUS ELECTROLUMINESCENT LAYER ELECTRICALLY CON NECTED TO AND INTERPOSED BETWEEN FIRST AND SECOND ELECTRICALLY CONDUCTIVE FILMS. 